1. Field of the Invention
The present invention relates to a vehicle air conditioner which automatically determines a windshield-fogging condition and automatically selects a foot/defroster mode to prevent a windshield from being fogged during heating operation in winter.
2. Description of Related Art
In a conventional automatic control type air conditioner for a vehicle, generally, heating operation for a passenger compartment is started from a foot mode in winter. During the foot mode, a large amount of air to be blown into the passenger compartment (e.g., about 70 to 80%) is blown toward the foot area of a passenger in the passenger compartment from a foot outlet, and a small amount of air is blown toward a windshield from a defroster outlet.
At a cold time when outside air temperature decreases to 0.degree. C. or lower, if only a small amount of air (warm air) is blown toward the windshield from the defroster outlet, the temperature of the windshield is not sufficiently increased, and the inner surface of the windshield is readily fogged. Accordingly, a driver (passenger) manually switches an air-blowing mode to a foot/defroster mode or a defroster mode to defrost the windshield.
In this case, whenever the windshield is fogged, it is necessary for the driver to manually set the foot/defroster mode or the defroster mode, and the manual operation annoys the driver.
U.S. Pat. No. 5,137,213 discloses a vehicle air conditioner for improving both defrosting performance for a windshield and heating performance for a passenger compartment. That is, a first defroster/foot mode corresponding to the foot mode and a second defroster/foot mode corresponding to the foot/defroster mode are set as a defroster/foot mode where air is blown toward the inner surface of the windshield from the defroster outlet while air is blown to the foot area (lower side) from the foot outlet. In the first defroster/foot mode, an amount of air blown from the defroster outlet is made smaller. In the second defroster/foot mode, the amount of air blown from the defroster outlet is made larger than that during the first defroster/foot mode. When the outside air temperature is low, the windshield is prevented from being fogged by setting the second defroster/foot mode. When a target temperature of air blown into the passenger compartment is set higher than a predetermined temperature, the air-blowing mode is switched to the first defroster/foot mode while air-blowing amount is increased in accordance with an increase of the target temperature.
However, since the air-blowing mode is switched between the first defroster/foot mode and the second defroster/foot mode) according to only the conditions of the target temperature of air blown into the passenger compartment and the outside air temperature, a windshield-fogging condition cannot be accurately determined, and the windshield may be fogged.
On the other hand, if a setting range of the foot/defroster mode is simply enlarged in order to improve the fog-preventing performance of the windshield, a setting range of the foot mode is relatively reduced, thereby deteriorating the heating performance of the passenger compartment.